Glial cells play critical roles in the nervous system. Rather than being passive support cells as long thought, they are highly active participants. Recent work has shed new light on their many functions, include regulation of synapse formation and function, control of neural circuits, and neuro-immune interactions. It is also shedding light on the part they play in neurodegenerative diseases and malignancies such as glioma, as well as the process of axonal regeneration and CNS repair.